III. Subject Selection A. Inclusion/Exclusion Criteria: Through a separate protocol, Dr. Newton-Cheh has recruited to date over 1000 healthy volunteers aged 18 to 40 with consent for re-contact on the basis of genotypes determined after an initial screening visit, all of whom have undergone DNA extraction. From the existing collection of 1000 subjects, approximately 75% are self-described European/Caucasian and 8% are African Americans. These are otherwise healthy individuals ages 18 and older. For this study, we will genotype 68 independent QT SNPs using the Sequenom genotyping array in the MGH Center for Human Genetic Research Sequenom core on these samples. The genotype score is the sum of the predicted effects on QT interval for each genotype. For example, SNP rs12143842 is a C/T SNP; for each copy of the T allele the QT interval is 3.50 msec higher such that individuals with the TT genotype have a QT interval 7 msec higher than those with CC genotype and individuals with the CT genotype have a QT interval 3.5 msec higher. These predicted effects are then determined for all 68 SNPs and summed to create a single number, the predicted change in QT interval on the basis of those genetic variants. From the genotype score, we will determine the set of individuals who belong to the top or bottom quintile of QT genotype risk score to be eligible for our moxifloxacin study (see below for details of the genotype score calculation). Apparently healthy male and female volunteers of European and African American ancestry, between 18 and 50 years old, will be eligible if they are free of known cardiovascular, renal, hepatic disease, have no personal or family history of sudden cardiac death; use no prescribed or over-the-counter medications as well as recreational drugs; have no bradycardia (defined as resting heart rate < 50 bpm), conduction disease (QRS > 100ms) or QTc prolongation on electrocardiography (QTc > 500msec); and have a normal serum potassium (K>3.3) and magnesium (Mg>1.8), as well as renal and liver function tests. We will exclude woman who are nursing, pregnant or planning to become pregnant during the study period, counsel them on the importance of using birth control during the study period, and check a serum HCG on the screening visit. We estimate that 266 individuals (133 in the highest and lowest quintiles, respectively) will be eligible.